The present invention relates to the field of light emitting diode based lighting and more particularly to a method of improved color control for LED backlighting.
Light emitting diodes (LEDs) and in particular high intensity and medium intensity LED strings are rapidly coming into wide use for lighting applications. LEDs with an overall high luminance are useful in a number of applications including backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a matrix display. In a large LCD matrix display typically the LEDs are supplied in one or more strings of serially connected LEDs, thus sharing a common current. Matrix displays typically display the image as a series of frames, with the information for the display being drawn from left to right in a series of descending lines during the frame.
In order supply a white backlight for the matrix display one of two basic techniques are commonly used. In a first technique one or more strings of “white” LEDs are utilized, the white LEDs typically comprising a blue LED with a phosphor which absorbs the blue light emitted by the LED and emits a white light. In a second technique one or more individual strings of colored LEDs are placed in proximity so that in combination their light is seen as a white light. Often, two strings of green LEDs are utilized to balance one string each of red and blue LEDs.
In either of the two techniques, the strings of LEDs are in one embodiment located at one end or one side of the matrix dispaly, the light being diffused to appear behind the LCD by a diffuser. In another embodiment the LEDs are located directly behind the LCD, the light being diffused, so as to avoid hot spots, by a diffuser. In the case of colored LEDs, a further mixer is required, which may be part of the diffuser, to ensure that the light of the colored LEDs is not viewed separately, but rather are mixed to give a white light. The white point of the light is an important factor to control, and much effort in design in manufacturing is centered on the need to maintain a correct white point.
Each of the colored LED strings is typically intensity controlled by both amplitude modulation (AM) and pulse width modulation (PWM) to achieve an overall fixed perceived luminance. AM is typically used to set the white point produced by the disparate colored LED strings by setting the constant current flow through the LED string to a value achieved as part of a white point calibration process and PWM is typically used to variably control the overall luminance, or brightness, of the monitor without affecting the white point balance. Thus the current, when pulsed on, is held constant to maintain the white point among the disparate colored LED strings, and the PWM duty cycle is controlled to dim or brighten the backlight by adjusting the average current. The PWM duty cycle of each color is further modified to maintain the white point, preferably responsive to a color sensor, such as an RGB color sensor. The color sensor is arranged to receive the mixed white light, and thus a color control feedback loop may be maintained. It is to be noted that different colored LEDs age, or reduce their luminance as a function of current, at different rates and thus the PWM duty cycle of each color must be modified over time to maintain the white point set by AM.
One known problem of LCD matrix displays is motion blur. One cause of motion blur is that the response time of the LCD is finite. Thus, there is a delay from the time of writing to the LCD pixel until the image changes. Furthermore, since each pixel is written once per scan, and is then held until the next scan, smooth motion is not possible. The eye notices the image being in the wrong place until the next sample, and interprets this as a blur or smear.
This problem is addressed by a scanning backlight, in which the matrix display is divided into a plurality of regions, or zones, and the backlight for each zone is illuminated for a short period of time in synchronization with the writing of the image. Ideally, the backlighting for the zone is illuminated just after the pixel response time, and the illumination is held for a predetermined illumination frame time whose timing is associated with the particular zone.
An additional known problem of LCD matrix displays is the lack of contrast, and in particular in the presence of ambient light. An LCD matrix display operates by providing two linear polarizers whose orientation in relation to each other is adjustable. If the linear polarizers are oriented orthogonally to each other, light from the backlight is prevented from being transmitted in the direction of the viewer. If the linear polarizers are aligned, the maximum amount of light is transmitted in the direction of the viewer. Unfortunately, a certain amount of light leakage occurs when the polarizers are oriented orthogonally to each other, thus reducing the overall contrast.
This problem is addressed by adding dynamic capability to the scanning backlight, the dynamic capability adjusting at least one of the overall luminance and the color balance of the backlight for each zone responsive to the current video signal. Thus, in the event of a dark scene, the backlight luminance is reduced thereby improving the contrast. It is further expected that in certain conditions the color balance may be further adjusted responsive to the current video signal, thereby improving the color range. Since the color, and overall luminance, of a scene may change on a frame by frame basis, the color control feedback loop must rapidly respond to changes in desired color and/or luminance. In such an embodiment the color control feedback loop must feed back and control the color balance and luminance. It is to be noted that a new frame begins every 16.7-20 milliseconds, depending on the system used.
The prior art teaches that samples of the LED backlighting be passed through a low pass filter (LPF) exhibiting a frequency cutoff 40 dB less than the PWM frequency, i.e. 1/100 of the PWM frequency. Thus, for a PWM frequency on the order of 2 kHz an LPF exhibiting a cutoff frequency of 20 Hz is taught which is lower than the frame cycle time. Thus, there is no opportunity to correct the LED color during a frame. The above is further compounded by the fact that the LEDs are only enabled for approximately ¼ of the frame.
U.S. Pat. No. 6,894,442 issued May 17, 2005 to Lim et al is addressed to a light source and a method for controlling same. Lim provides for a low pass filter, whose response is long in relation to the PWM period. When a target light value is changed, a control signal is initially replaced by a predicted control signal based on the new target value, rather than the error signal generated in a servo. The need to generate and store predicted control information adds to cost.
What is needed, and not provided by the prior art, is a means for operating a feedback color loop of a PWM controlled light source whose target value may be changed on a frame by frame basis.